This invention relates to a reactor for continuous gasification of fuels which are mainly in the form of lumps, under superatmospheric pressure, in a water-cooled double-walled reactor chamber, by a treatment with a gasifying agent consisting of gases that contain free oxygen in a mixture with saturated or superheated water vapor and, if desired, other gases.
This invention constitutes a further development of the process and apparatus disclosed in U.S. Pat. No. 3,937,620. Further details of the pressure gasification of solid fuels and of the reactor required for that purpose are known from U.S. Pat. Nos. 2,667, 409; 3,930,811; and 3,902,872; and Printed German Application 1,021,116.
The composition of the product gas which is produced in the reactor depends in high degree on the composition of the gasifying agent.
The lower limit of the proportion of stream to be admixed with the free oxygen depends on the sintering and melting behavior of the ash contained in the fuel which is to be gasified.
Such reactors normally contain in their lower portion a substantially conical grate which is rotatably mounted and serves to discharge the gasification residue, which consists of ash in lump and/or granular form. The grate serves also to introduce the gasifying agent into the reactor shaft. The gasifying agent is normally supplied and distributed through a plurality of concentric annular slots in the top of the grate. A further distribution of the gasifying agent throughout the cross-section of the reactor shaft is accomplished by the ash bed lying on the top of the grate. The distribution will be improved by an ash bed having a uniform particle size and thickness.
The gasifying agent flowing through the ash bed takes up part of the sensible heat of the ash. This is beneficial for the gasification.
Any disturbance arising in the ash bed, e.g., as a result of a discharge of ash at an excessively high or excessively low rate, or an increase or decrease of the particle size of the ash, etc., will immediately affect the gasification.
It has been found in operation that the particle size of the ash depends not only on the composition of the gasifying agent but also on the distribution of the gasifying agent in the combustion zone of the reactor.
The use of the previously known grates did not result in an optimum distribution of the gasifying agent throughout the shaft area but in a preferential supply to the central region of the shaft. The increased supply of gasifying agent to the central region of the shaft results in a more intense combustion in that region so that the highest combustion temperatures which can be reached in theory are more closely approximated and the formation of slag is thus promoted whereas the composition of the fuel ash and the melting and sintering behavior of such ash are not changed.
On the other hand, the annular portion of the combustion zone near the shaft wall is suplied with less gasifying agent and is more intensely cooled. Fuel which has not been gasified can travel along the shaft wall to a region which is closely above the grate and from the latter region into the deadburnt ash thereby being lost.
This phenomenon also has an influence on the rate at which ash is discharged, with repercussions on the gas production rate and the composition of the product gas. For instance, when a formation of slag has resulted in a retention of ash, the grate may be rotated at a higher speed to crush the ash and the discharge of crushed slag may be suddenly succeeded by a discharge of ash from the reactor shaft at an excessively high rate. In that case the core of the combustion zone will descend too close to the grate so that the grate is locally overheated and may be damaged. In any case, the distribution of the gasifying agent leaving the top of the grate will be even less uniform so that any irregularities, such as an inclination of the surface of the ash bed, or a generation of steam in the jacket at a high and fluctuating rate, will be intensified. The output of the reactor will then decrease for hours, and the proportion of unburnt fuel in the ash will rise steeply, whereas the carbon dioxide content in the product gas will increase at the expense of its combustion constituents. The temperatures at the gas outlet of the reactor will also be higher than normal. In that case there is a danger of a channeling of free oxygen.
High gas outlet temperatures and slag-clogged grates often require an interruption of operation.
Because difficulties of that kind may arise, the operators must be highly attentive and must be highly skilled so that they can recognize the position and state of the combustion zone within the reactor. The structural alterations which have been adopted in the past have not basically improved the performance of the gasification process.